In general poems do not have “correct” scansions. Two readers who understand a line in different ways may put the stress on different words, leading to different scansions. So there is always a degree of personal opinion, meaning that Fussell might think the lines too regular, while you find a pleasant amount of variation. Nonetheless, there are a few points in the question that do not seem quite right to me.
First, the question suggests that particular words or sequences of words should always be scanned the same way (viz. “‘too much’ appears in another quoted poem as well and has been marked as a spondee”). This is not the case: the same words may be scanned one way in one context and another way in a different context. C. S. Lewis gave this demonstration of the phenomenon:
I have given no man of my fruit to eat,
I trod the grapes, I have drunken the wine.
Had you eaten and drunken and found it sweet,
This wild new growth of the corn and vine;
and now read this:
I comfort few and many I torment,
Where one is spared a thousand more are spent;
I have trodden many down beneath my feet,
I have given no man of my fruit to eat.
I conjecture that you have read the last line of my second example differently from the opening line of my first: yet as mere language, separated from the ideal pattern, they are identical.
C. S. Lewis (1939). ‘The Fifteenth-century Heroic Line’. In Laurence Binyon, ed. (1939). Essays and Studies, volume XXIV, p.31. Oxford: Clarendon.
So if both syllables of “too much” are stressed in one line, it does not follow that they must be stressed in all lines. The scansion is affected by the context, so that it’s necessary to look at each line in its own context.
Second, the question analyzes line 3 as “three anapests and two iambs” but a careful count of the syllables shows that’s not right: there are four syllables in “-sent is to make” so this would be a fourth paeon and not an anapest; and similarly for “-y of the Pre-” (you could imagine eliding “-y” and “of”, but it would be a bit stilted). However, English verse does not really have paeons, because there is always some variation in stress on adjacent syllables, so that a fourth paeon turns into a pair of iambs by the addition of a little stress on the second syllable, a so-called “promoted” stress.
Third, if the context of a line is highly regular, many (most?) readers will do their best to apply the same kind of regularity to the line itself. That’s what’s going on in the quotation from C. S. Lewis above, where the first quatrain is anapestic tetrameter with a few substitutions by iambs, so many readers will read “I have given no man of my fruit to eat”, and the second quatrain is iambic pentameter, so many readers will read “I have given no man of my fruit to eat”, with a promoted stress on “of”.
Putting all this together, in the context of ‘America For Me’, by the time the fifth stanza in reached, the mostly-iambic context is well established, so that many readers, including Fussell and myself, read the lines as
The Past is too much with her, and the people looking back.
But the glory of the Present is to make the Future free—
where the first line has a promoted stress on “and” and the second line is headless and there is a promoted stress on “of”. Whether this is too regular is a matter of taste. One might say that a metrical regularity suits the bombast.